The present invention relates generally to compact broadband antennas and, more particularly, to compact, electrically-small broadband antennas for field generation applications that are derived from an inverted-F antenna configuration. The present invention also relates to these antennas when further combined with antennas derived from a log-periodic array antenna configuration for extending the bandwidth of the combined configuration.
In many applications, and in particular Electromagnetic Compatibility (EMC) testing applications, it is desirable to have a compact broadband antenna for generating high intensity electric fields. A typical application may require an antenna that is capable of generating an electric field intensity of 20 volts per meter over a frequency range of from 20 MHz to 2 GHz at a distance of 3 meters in front of the antenna using an amplifier having the least possible power to feed the antenna. At the low end of this frequency range, a compact antenna will necessarily be electrically small. (An antenna is commonly considered to be electrically small if it fits inside a sphere of radius 1/2 .pi. wavelengths of the lowest frequency of interest. An antenna with a lower frequency of interest of 20 MHz would be electrically small if it fit inside a sphere with a radius of less than 2.4 meters. A practical manageable size for an antenna that must be set up for a test and subsequently disconnected and removed from a test chamber is about 1.5 meters in extent.) Unfortunately, electrically small antennas are inherently narrowband and inefficient, so that design of a compact antenna, in either a conventional or hybrid combination configuration, having high electric field intensities over a useful range of from 20 MHz to 2 GHz has eluded those skilled in the field of antenna design.
Frequency-independent antennas, and in particular, log-periodic dipole arrays (LPDAs) are widely used for applications requiring the generation of broadband electromagnetic fields. However, at the lower end of their operating frequency range (the frequency range over which they exhibit frequency-independent behavior), these antennas must be approximately one-half of a wavelength in width. Thus, an LPDA with a lower operating frequency of 30 MHz (10 meter wavelength) must be approximately 5 meters wide. Because such dimensions are unacceptably large and because operating frequency ranges extending from 20 MHz to 2 GHz are required by the electromagnetic compatibility (EMC) testing industry, techniques for reducing the size of hybrid antennas have been sought. One commonly used technique to extend the frequency range of a LPDA downward is to use a broadband dipole to replace the lowest frequency element in a LPDA. For example, a bowtie dipole element is sometimes used in conjunction with a 150 MHz LPDA to extend the low frequency response of the antenna down to 30 MHz. Another possibility is to use a biconical dipole element to replace the lowest frequency element. However, none of these configurations are capable of achieving a usable bandwidth from about 20 MHz to about 2 GHz required for EMC testing without being driven by prohibitively large amplifiers. Examples of LPDAs are described in the following U.S. Pat. Nos.: 4,754,287 and 5,057,850.
Size reduction in linear antennas has been obtained by "folding" a vertical resonant quarter-wave monopole antenna into an "inverted-L" shape. The length needed for resonant operation is thus obtained as the sum of the two dimensions, height and length, as opposed to just height. While the inverted-L antenna is a useful reduced-size antenna, it suffers from low input impedance and thus requires an external matching network which adds complexity and reduces efficiency and power handling capability. The overall length allows the antenna to resonate and thus provide zero input reactance. However, the reduced height causes the radiation resistance at the fundamental resonance to be unacceptably small.
The inverted-F antenna is essentially a tapped inverted-L antenna. That is, the antenna is tapped at some non-zero distance from the base in order to provide a broadband impedance transformation. This impedance transformation is derived from the current distribution in the antenna which can be assumed to vary in a sinusoidal manner along the arc length as measured from the base. Thus, any impedance level may be obtained by adjusting the tap position. The inverted-F antenna is superior to the inverted-L antenna because it requires no external matching network and allows independent adjustment of the resonant frequency of the antenna and impedance level. In general, the overall length of the inverted-F antenna (height plus length) must be approximately one-quarter wavelength at the resonant frequency. The planar inverted-F antenna (PIFA) is a derivative of the inverted-F antenna in which the top wire is replaced with a plate. This has been shown to lower the radiation Q and hence broaden the frequency response of the antenna while still retaining the desirable characteristics of the wire inverted-F antenna: no required external impedance transformer and independent adjustment of resonant frequency and impedance level. While having many desirable characteristics, the planar inverted-F antenna still lacks the requisite bandwidth to allow implementation of a compact hybrid antenna for EMC testing. Examples of PIFAs are described in the following U.S. Pat. Nos.: 5,365,246 and 5,434,579.
While currently available hybrid antennas are superior to LPDA antennas alone, they are still ineffective at the low-frequency end of their operating range. This unsatisfactory performance at the low-frequency end imposes severe amplifier requirements for electric field generating systems. Because the amplifier is generally the most costly component of the system, relaxing amplifier performance requirements would have a significant effect on system cost.
For the foregoing reasons, there is a need for a small size antenna capable of generating high electric field intensities over the frequency range of from 20 MHz to 2 GHz. Specifically, there is a need for an antenna that is capable of achieving the desired electric field intensity of at least 20 volts per meter at a distance of 3 meters in front of the antenna using an amplifier having an output power of less than 500 watts to feed the antenna. Because of practical considerations, the dimensional extent of the antenna should not exceed 1.5 meters. In order to achieve a small size while maintaining a wide bandwidth, a broadband, electrically-small antenna can be mated with a frequency independent antenna. The primary problem is to obtain the necessary bandwidth from the electrically-small antenna. The secondary problem is to properly mate the electrically-small antenna to the LPDA or other frequency-independent antenna. That is, the frequency characteristics of the antennas should "cross-over" in such a way that destructive interference does not occur. It is also important that coupling between the antennas does not detract from the beam pattern of the LPDA or other frequency-independent antenna.